Clarity of water is usually measured by its turbidity. Turbidity is a measure of the extent to which light is either absorbed or scattered by suspended material in water, but it is not a direct quantitative measurement of suspended solids. Both the size and surface characteristics of the suspended material influence absorption and scattering.

Turbidity measurement is an important factor related to the quality of drinking water. It should be measured in treated wastewater effluent if it is reused. If ultraviolet radiation (UV) is used for disinfection of treated wastewater, turbidity measurement will be important because for UV to be effective in disinfecting wastewater effluent, UV light must be able to penetrate the stream flow.

Turbidity is measured by comparing the intensity of light scattered by the sample with the intensity of light scattered by a standard solution. The results are reported in NTU.